Field of the Invention
The invention relates in general to a composition of a solution type styrene butadiene rubber (SBR), and more particular to a rubber suitable for a tread stock of a tire.
The composition of the rubber of the invention basically includes; at least one 10%.about.50% weight percentage low molecular weight coupled copolymer of aromatic vinyl compound and conjugated diene or conjugated diene monopolymer, at least one 10%.about.60% weight percentage middle molecular weight linearly uncoupled copolymer of aromatic vinyl compound and conjugated diene, and at least one 5.about.60% weight percentage high molecular weight coupled copolymer of aromatic vinyl compound and conjugated diene. The coupled site of the rubber composition is coupled by the compounds of tin, phosphorous, gallium and silicon. The central atoms consist of tin, phosphorous, gallium and silicon. The rubber of the invention has improved processability and is superior in milling and mixing in a Banbury mixer, and also has a lower rolling resistance, which therefore results in lower consumption of gasoline and diesel oil, and higher wet traction. The elastomer of the invention is suitable as the rubber for tread stock, especially for the racing tire and the pneumatic tire.
Some conventional solution type SBR fabricated by continuous processes have broader molecular weight distribution. This solution type SBR is limited to the reactive speed ratio of styrene to butadiene so that the vinyl group content in the microscopic structure of the butadiene monomer is hard to change. Also, most of the terminal functional groups of this solution type SBR are not modified so that this SBR will not be able to uniformly mix with the carbon black in the process of masticate. Moreover, the bonding of carbon black is weak and is not be able to satisfy the conflicting requirement of both low rolling resistance and high wet traction. On the other hand, solution type SBR fabricated by batch type processes have narrower molecular weight distribution. This SBR includes uncoupled styrene/butadiene copolymers and coupled styrene/butadiene copolymers with tin, phosphorous, gallium, and silicon as coupling center. Due to the narrower molecular weight distribution of this copolymers, this solution type SBR tends to congregate during the process of mastication, using mill and Banbury mixer. Consequently, the application of the solution type SBR is limited. However, the present invention has superior processability as using mill and Banbury mixer and satisfies the conflicting requirement of low rolling resistance and high wet traction.
The term, "amount of vinyl group", indicates the weight percentage of conjugated diene unit polymerized at the position 1, 2 or 3, 4 in this invention. For example, the amount of vinyl group for 1,3-butadiene means the ones at the position 1, 2. For an isoprene, it includes the total amount of the vinyl group at the position 1, 2 and 3, 4.
The method disclosed in U.S. Pat. No. 3,980,625, utilizing the solution rubber of the reflux portion in a reactor on a continuous type process to increase the distribution of the molecular weight and consequently eliminate the cool flow and improve processibility. Conventionally, several liquid rubber with different Mooney viscosity are mixed and also they have to be reacted in separated reactors before being mixed. The method disclosed in this patent overcomes the disadvantage in the mixture process for mixing the liquid rubber with different Mooney viscosity. However, the terminal functional group is not modified and the coupling agent is not utilized in this invention so that the rubber of this patent is not compatible with the carbon black. This rubber is therefore not suitable for tire tread. However, the rubber of the present invention has a broader molecular weight distribution. Also, the branch type SBR with tin, phosphorous, gallium, or silicon as coupling center of the invention overcomes the difficulties in processing and strengthens the compatibility and bonding with carbon black.
In the background description of the U.S. Pat. No. 4,866,131, it is stated that a large amount of aromatic oil, as a softening agent, are is added for producing the high-performance tire to increase the road gripping ability and obtain higher tan .delta. (the hysteresis loss value). This method is unfavorable because some important characteristics of tire tread such as rupture strength, wear resistance and blow out resistance are significantly marred by the increased softening agent. It is also mentioned that conventionally, for producing high-performance tire of racing tire, high-styrene styrene-butadiene copolymer rubber formed by emulsion polymerization is utilized to obtain higher hysteresis loss. However, this rubber has poor vulcanizability, low rupture strength and low rupture resistance. The solution type conjugated diene/aromatic vinyl compound copolymer of the present invention overcomes these drawbacks. In the U.S. Pat. No. 4,866,131, low molecular weight copolymer of an aliphatic diene and an aromatic vinyl compound (the molecular weight in the range of 2000 to 50000) is used in a hydrocarbon solution instead of the conventional aromatic oil. However, a large amount of the initiator has to be consumed and a large amount of inorganic compound such as lithium chloride and hydroxy lithium produced after the reaction is not able to solve in the solvent. Moreover, an additional reactor is required and the copolymer has to be blended with the styrene-butadiene rubber in another tank. However, the solution type conjugated diene/aromatic vinyl compound copolymer of the present invention contains this low molecular weight copolymer and the reaction can be performed in one reactor without the step of blending and the problem of producing a large amount of insoluble organic compounds, which overcomes the drawbacks of the U.S. Pat. No. 4,866,131.
In the U.S. Pat. No. 4,940,756, a mixture consisting of two different copolymer compositions of the conjugated diene/aromatic vinyl compound is disclosed. Theses two different copolymer block compositions have two different values of glass transition temperature (Tg) so that the mixture satisfies the requirement of low rolling resistance and high wet gripping ability. Within the mixture, a copolymer block of conjugated diene and aromatic vinyl compound has a Tg value higher than -55.degree. C. and another copolymer block of conjugated diene and aromatic vinyl compound has a Tg value lower than -65.degree. C. The block with a Tg value higher than -55.degree. C. contributes the rubber the characteristics of high wet gripping ability together with high rolling resistance and low abrasion resistance. The other block with a Tg value lower than -65.degree. C. provides contrary properties. The block with a Tg value higher than -55.degree. C. contains high amount of aromatic vinyl compound and the vinyl in the conjugated diene unit is higher. The other block with a Tg value lower than -65.degree. C. contains low amount of aromatic vinyl compound and the vinyl in the conjugated diene unit is lower. Due to the incompatibility of theses two blocks and the narrow distribution of the molecular weight of their mixture, the block mixture tends to stick on the kneader and the blade of the Banbury mixer during the processes of vulcanization and mastication. Moreover, during the second and the third periods of the reaction, gradual additions of the monomers have to be performed to maintain the random arrangement of conjugated diene and aromatic vinyl compound. Consequently, the vinyl content in conjugated diene unit can not be adjusted and it takes long to polymerize. However, the process of the invention makes the vinyl content in the momomer of conjugated diene and aromatic vinyl compound adjustable in each block. Also, a single and quick addition of monomer is acceptable by the present invention and the copolymer of the present invention has superior processibility.
In the U.S. Pat. No. 4,396,743, a method using a wider distribution of Tg in tan .delta.-temperature curve with a equal Tg is proposed. This method provides the rubber the properties of low rolling resistance and high wet traction but not be able to overcome the problem of poor processibility resulted from the narrow distribution of molecular weight.